An electronically non-conducting, ion-conducting medium is an essential part of fuel cells and electrolyzers, can be used in separations and ion exchange, and is also important for other electrochemical applications. Liquids can serve as the ion-conducting medium but there are practical difficulties with the use of liquids and it is highly advantageous to use ion-conducting membranes instead of liquids for electrochemical applications. In all fuel cells and electrolyzers where water is involved, the fundamental reactions require either a proton-conducting membrane or anion-conducting membrane. Of the two, proton-conducting membranes, which operate in acidic media, offer the required combination of adequate longevity and good conductivity at near ambient temperature (25-100 degrees Celsius).
The aqueous reduction chemistry that takes place in acidic media is summarized by the equation½O2+2H++2e−→H2O.Precious metals such as platinum and platinum alloy catalysts are used as electrocatalysts in acidic media because they are relatively unreactive with strong acids and can perform the needed electrocatalytic oxidation and reduction chemistry in acid media. However, the above reduction reaction is relatively slow using a platinum catalyst and requires an overpotential on the order of about 0.3 to 0.5 volts to drive the reaction at a useful rate.
While fuel cells and electrolyzers employing acidic electrolytes require proton-conducting membranes, those employing alkaline electrolytes require anion-conducting (hydroxide-conducting, in particular) membranes. The aqueous reduction chemistry that takes place in alkaline media is summarized by the equation½O2+H2O+2e−→2OH−.This reduction reaction is relatively rapid compared to the previous reaction and requires a smaller overpotential (about 0.2-0.4 volts) to drive the reaction at a useful rate, which also results in producing less waste heat for removal. Besides the increased efficiency of the oxygen reduction reaction, the expensive noble metal catalysts that are required for proton-conducting systems could be replaced with inexpensive base metal catalysts such as nickel.
Attempts have been made at identifying and developing materials useful for hydroxide-conducting membranes. Examples of these materials include cationic polymers with tetraalkylammonium and N-alkylpyridinium side chains and mobile hydroxide counterions. The chemical stability of these polymers has been reported in “Ionic Polymers VI. Chemical Stability of Strong Base Anion Exchangers in Aggressive Media,” Polymer Degradation and Stability, vol. 70 (2000) pp. 463-468, incorporated by reference herein. According to this paper, exposing these polymers to concentrated alkali results in chemical degradation of the side chains. Some of the degradation chemistry for one of these polymers in concentrated alkali is shown SCHEME 1 below.
Pathway ‘a’ of SCHEME 1 shows polymer degradation by demethylation of the starting cationic polymer. This pathway produces a neutral polymer with an amine side chain and a molecule of methanol. Pathway ‘b’ shows a degradation that generates polymer with a benzylic alcohol side chain; a molecule of a neutral amine is also released. Pathway c shows a beta-elimination degradation. The beta-elimination of the starting cationic polymer results in the production of a neutral polymer with an amine side chain. An olefin molecule is also produced.
The use of anionic-conducting membranes for fuel cells has been reviewed by Varcoe et al. in “Prospects for Alkaline Anion-Exchange Membranes in Low Temperature Fuel Cells,” Fuel Cells, vol. 4, no. 4 (2004), pp. 1-14, incorporated by reference herein. According to this paper, anion-conducting membranes suffer from performance loss when they are used in the air. While the reasons for the loss in performance are not fully understood, it is believed that the degradation of the polymer and the formation of carbonates play an important role.
Anion-conducting membranes, hydroxide-conducting membranes in particular, that are chemically stable, highly conducting, and not significantly affected by degradation or by the formation of carbonates remain desirable.
Therefore, an object of the present invention is an anion-conducting membrane that is more stable to chemical degradation at high pH than currently available anion exchange membranes.
Another object of the present invention is a polymer that may be used to prepare a robust anion-conducting membrane.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.